1. Field of the Invention
The present invention relates to a cutting tool having a flat drag portion and to a holder-carrying tool for carrying a holder to which a cutting tool is fixed.
2. Description of the Related Art
In a cutting process according to the related art, ridges and valleys (ridges and valleys representative of a roughness of a processed surface) of a surface to be finished, which are called feed marks, are formed in some cases on and in a surface of a material to be cut, as shown in FIG. 6(a), due to interaction between radiused nose R of a corner part (nose portion) of a cutting tool and the feeding of the material.
Therefore, in order to lower the roughness of the surface to be finished, by removing the feed marks (smooth the surface), a cutting tool provided with a widthwise cutting edge (flat drag edge) parallel to the surface to be finished is used mainly in a face milling process as shown in FIG. 6(b).
The above-mentioned cutting tool for smoothing the surface is formed of, for example, a ceramic tool. Since the tenacity of a ceramic tool is low as compared with that of a cermet tool or a carbide tool, a negative tip in which a rake face and a flank form a right angle is generally used.
When the negative tip is fixed to a front end of a holder, a rake angle (a front rake angle formed by inclining the front end portion of the holder, and a lateral rake angle formed by inclining a lateral portion of the holder) is provided on the holder so as to obtain sharpness of an edge by inclining the negative tip as shown in FIG. 7(a).
However, in this case, the flat drag forms a negative angle with respect to a surface to be finished as shown in FIG. 7(b), i.e., the flat drag is disposed in a sharp projecting state with respect to the surface to be finished. Therefore, the roughness of the surface to be finished cannot be lowered, so that a desired dimensional accuracy cannot be obtained.
Namely, though the flat drag is provided, the feed marks cannot be removed sufficiently.
The present invention addresses these problems, and aims at providing a cutting tool and a holder-carrying tool which are capable of lowering the roughness of a surface to be finished (smoothing the surface), during a cutting process.
According to an aspect of the present invention, a cutting tool is provided which has a flat drag between a curved portion of a nose portion of the tool and a flank, wherein the flat drag is provided on the inner side of a tangent at a terminal end section of the curved portion of the nose portion.
Preferably, a flat drag angle xcex8 formed by the tangent at the terminal end section of the curved portion of the nose portion with the flat drag is set to a level in excess of 0xc2x0.
According to the invention, the flat drag is not formed in such a manner as in the cutting tool of related art, i.e., it is not formed in the direction of a tangent at a terminal end section of a nose portion but provided so that the flat drag is on the inner side of the tangent thereof as illustrated in FIG. 1. Therefore, when the cutting tool is fixed to a holder, the flat drag becomes substantially parallel to the surface to be finished of a material to be cut, even when a front rake angle or a lateral rake angle are provided on a fixing surface (to which the cutting tool is to be fixed) of the holder. Accordingly, even when a cutting process is carried out by using such a tool, it is hard for ridges is and valleys (feed marks) to occur on the surface to be finished. This enables the roughness of the surface to be finished to be lowered, and the dimensional accuracy to be improved.
The terminal end section mentioned above is a position in which a curve having a radius of curvature of the nose portion terminates, from which position a planar flank forming a side surface of a tip is formed when a flat drag is not provided.
Preferably, the flat drag angle xcex8 made by the tangent with the flat drag is set in the range 0 less than xcex8 less than 5xc2x0.
According to this preferred feature, the range of the flat drag angle xcex8 is defined as shown in FIG. 1. The reason why the flat drag angle xcex8 is set in excess of 0xc2x0 resides in that the ridges and valleys of the surface to be finished can be removed as mentioned above as long as the flat drag angle is in this range.
In the meantime, the reason why the flat drag angle xcex8 is set to lower than 5xc2x0 resides in that, when the flat drag angle xcex8 is not smaller than 5xc2x0, the flank is shaped so that the flank excessively enters the inner side of the cutting tool since the flank extends from the flat drag further inward. This makes it difficult to fix the cutting tool to a holder.
A more preferable range is 0 less than xcex8xe2x89xa61xc2x0.
Preferably, a width (w) of the flat drag viewed from the rake face is set to 0 less than wxe2x89xa61.5 mm.
The width (w) of the flat drag is defined as illustrated in FIG. 1. When the width of the flat drag exceeds 1.5 mm, a cutting resistance becomes excessively large, and there is the possibility that the dimensional accuracy of a material to be cut is not obtained.
A more preferable range is 0.3 less than wxe2x89xa60.7 mm.
Preferably, the flat drag viewed from the rake face has an outwardly projecting curved shape or a linear shape.
This invention shows an example of the shape of the flat drag, and, as long as this shape is an outwardly projecting curvilinear shape or a linear shape, i.e. not a concave shape, the ridges and valleys of the surface to be finished can be removed suitably.
Preferably, the cutting tool is formed of a ceramic material or CBN.
These are examples of preferred materials for the cutting tool.
Hard materials can be named as the ceramic materials, which include alumina, alumina-TiC and silicon nitride, and which include besides these materials hard materials, such as cermet and the like. CBN is cubic boron nitride which is an especially advantageous material for the cutting tool because it enables dry-cutting to be performed which reduces the expense, waste and environmental problems of using cutting fluid. A cutting tool made of a ceramic material is formed of a ceramic sintered body.
Preferably, the cutting tool is a negative tip cutting tool.
This preferred feature gives examples the shapes of the cutting tool. A negative tip in which a rake face and a flank form a right angle as shown in FIG. 2(a) is taken as an example. Besides the negative tip, there also exists a positive tip in which a rake face and a flank form an acute angle as shown in FIG. 2(b). It is desirable that this invention be applied to a negative tip.
According to a further aspect of the present invention, the above-described cutting tool is fixed to a holder.
This invention accordingly provides a holder-carrying tool in which the above-described cutting tool is fixed to a holder (for example, a tip seat at a front end of the holder).
Therefore, when a cutting process is carried out by using this holder-carrying tool, the ridges and valleys of a surface to be finished can be suitably removed.
Preferably, when the cutting tool is fixed to the holder having a rake angle of 0xc2x0 or a negative angle, a flat drag angle xcex8xe2x80x2 with respect to the surface to be finished is 0xe2x89xa6xcex8xe2x80x2 less than 5xc2x0.
This preferred aspect of the invention exemplifies an angle at which a cutting process is carried out by using the above-described cutting tool fixed to a holder. Namely, when a cutting operation is carried out by using a cutting tool having the above-mentioned flat drag angle xcex8 and fixed to a holder, the angle xcex8xe2x80x2 of the flat drag with respect to a surface to be finished of a material to be cut can be set to 0xe2x89xa6xcex8xe2x80x2 less than 5xc2x0, i.e., in a substantially parallel condition. This enables the ridges and valleys of the surface to be finished to be suitably removed.
The angle xcex8xe2x80x2 is not the above-mentioned flat drag angle xcex8 but an angle of the flat drag of the cutting tool fixed to a holder with respect to the surface to be finished.